Code cleanup

OPENDJ-1941 CR-6639 dsconfig: impossible to edit a persistit backend configuration Method for changes in backend configuration was incorrectly behaving as an add regardless of backend's current status. revise it to let changes go through to the backend and only act when backend is going from disabled to enabled.

Fix OPENDJ-1891 Can't setup opendj instance, because of memory problem. Simplified calculations on used memory for fallback case.

OPENDJ-1727 CR-6350 db-cache-percent default values in persistit backend probably too aggressive Available memory in Old Gen (Runtime as fallback) is now strictly checked for pluggable backends and a warning logged for JEB backends.